Interferon regulatory factor 5 (IRF5) is a transcription factor that is activated following Toll-like receptor (TLR) signaling. Genetic variants in IRF5 hav been robustly associated with systemic lupus erythematosus (SLE) susceptibility in both genome-wide and candidate gene association studies. While IRF5 is strongly and consistently associated with SLE, the mechanisms by which IRF5 predisposes to human SLE are still unclear. Our central hypothesis is that SLE-associated variants in IRF5 promote autoantibody formation and alter transcriptional patterns in human immune cells following TLR ligation, resulting in risk of SLE. We have previously shown that SLE-risk haplotypes of IRF5 are associated with autoantibody formation in both SLE patients and healthy autoantibody positive individuals. The HLA locus is strongly associated with SLE, and the MHC molecules encoded by this locus can direct the immune response against particular self-antigens. We propose that SLE-associated variants in MHC molecules present self-peptides derived from particular nuclear antigens, and then IRF5 variants cooperate in autoantibody production by enhancing activation of self-reactive B-cells directed against these antigens. In our preliminary data, we support this concept with evidence for a strong gene-gene interaction between IRF5 and HLA upon autoantibody formation in SLE patients (OR=3.42). IRF5 is a transcription factor, and it seems likely that risk variants should influence SLE risk by altering transcriptional profiles in immune cells. In preliminary data, we show that IRF5 risk variants result in differential binding to ISRE promoter elements in human cell lines. IRF5 translocation to the nucleus was greatly augmented in SLE patient monocytes, while little increased translocation was observed in NK and T cell lineages. These data support an important role for IRF5 variations in human monocytes. We have two Specific Aims: Aim 1: Define gene-gene interactions between HLA haplotypes and IRF5 genetic variants upon autoantibody traits in human populations. We will genotype 1500 SLE patients at the IRF5 and HLA loci to examine IRF5-HLA gene-gene interactions upon autoantibody traits. Additionally, we will study a unique cohort of individuals with SLE-associated autoantibodies who have a range of diagnoses, including asymptomatic people who are at risk of progression to SLE. Aim 2: Determine changes in transcriptional targeting of the IRF5 SLE-risk variants in monocytes and B cells from SLE patients, autoantibody positive asymptomatic subjects, and healthy controls. We will purify cells from the two lineages noted above from subjects homozygous for risk or protective haplotypes of IRF5. Cells will be stimulated with TLR7 or TLR9 ligands. ChIP-seq will be performed, and peaks will be compared between genotype categories and between patient groups. Our long-term goal is to understand the pathogenic mechanisms of human SLE, so that diagnostic and therapeutic approaches can be informed by the molecular biology of disease in a given individual. We will make progress toward this goal via the following expected outcomes: 1. Identify robust IRF5-HLA interactions that contribute to the break in tolerance to nuclear antigens observed in autoimmune disease and 2. Define transcriptional targets of IRF5 that are characteristic of the SLE-risk haplotypes and determine how these transcriptional patterns are associated with varying degrees and stages of autoimmunity in human populations (healthy, asymptomatic seropositive, and SLE).